The transsynaptic retrograde transport of the Bartha strain of pseudorabies virus (PRV Bartha) has become an important neuroanatomical tract-tracing technique for the characterization of neuronal circuits in the central nervous system. Recently, dual viral transneuronal labeling has been introduced by employing recombinant strains of PRV Bartha engineered to express different reporter proteins. Dual viral transsynaptic tracing has the potential of becoming an extremely powerful technique for defining interactions between parallel neural circuits in the brain. However, the current use of recombinant strains of PRV expressing different reporters that are driven by different promoters, inserted in different regions of the viral genome, and detected by different methods, limits the potential of these recombinant PRV Bartha strains as dual transsynaptic tracers. We have developed two isogenic recombinant strains of PRV Bartha (i.e., PRV152 and PRV614) differing only in the fluorescent reporter protein they express. PRV152 expresses the enhanced green fluorescent protein (EGFP), is driven by the human cytomeglovirus (CMV) promoter, and is inserted in the middle of the gG gene in the middle of the viral genome. PRV152 is in wide use and is well characterized. PRV614 expresses a novel monomeric red fluorescent protein (mRFP1) driven by the CMV promoter also inserted in the middle of the gG gene. It has only recently been constructed and is uncharacterized. The availability of two viral transneuronal tracers expressing different fluorescent reporters that can be visualized concurrently without additional tissue processing has enormous utility. In this application, we propose to characterize the newly developed PRV614 as a transsynaptic retrograde viral tracer that can be used in combination with PRV152 to further define neuronal circuits in the brain. The kinetics of PRV614 infection and retrograde transport will be determined in vivo using the retrograde transport of PRV through autonomic circuits innervating the eye. The ability of two isogenic strains of PRV to infect the same neuron when one PRV arrives later than the other will be determined both in vivo and in vitro. Transneuronal retrograde dual PRV labeling has the potential to be a powerful addition to the neuroanatomical tools for investigation of neuronal circuits; PRV 614 will eliminate many of the pitfalls associated with the currently used dual PRV recombinants. [unreadable] [unreadable]